In international relations (IR), “friend” may be an excessively naïve and even inappropriate term to use in research. Nevertheless, after the Cold War, IR scholars have gradually and consistently focused their attention on friendly international relations. Friendship is perhaps a key concept to understanding the current relationship between the United States and Japan after the Cold War. In discussing friendship, this opening chapter reviews the strengths and weakness of three theoretical perspectives regarding friendship: realism, liberalism, and constructivism. This chapter then contends that the security community is the most effective concept through which US-Japan relations can be comprehended. Finally, this chapter surveys past studies on the security community and states this book’s contribution by examining the case of US-Japan relations.